Abstract: Optical microscope, scanning electron microscope, transmission electron microscope, electron back scattering diffraction, X-ray diffraction and hardness measurement were used to investigate the microstructure of the different zones in nuclear grade 316LN welded joints. The results show that, the solidfication type of weld zone was dominated by ferrite-austenite(F-A) mode, and the content of vermicular-or lath-like residual ferrite existed in the austenite matrix was 13.66%. The fusion zone solidficated in the form of austenite-ferrite(A-F) mode, and the width of which was not uniform. Moreover, the fusion zone exhibited the epitaxial growth tendency. Recrystallization, grain growth and deformation behavior with different degrees occurred in the heat-affected zone. Meanwhile, less Σ3 grain boundary, high hardness and residual stress were found in the heat-affected zone.